The present invention relates to optoelectronic devices and methods of forming and operating same and, more particularly, to optoelectronic devices that utilize reflective surfaces to direct optical energy and methods of forming and operating same.
Micro-electromechanical (MEM) devices having mirrors therein have been proposed for directing optical beams across an optoelectronic substrate. Such devices are useful in a wide variety of applications ranging from displays to photonic NxN switches. Such devices are disclosed in U.S. Pat. No. 5,903,380 to Motamedi et al. entitled xe2x80x9cMicro-Electromechanical (MEM) Optical Resonator and Methodxe2x80x9d. In particular, the ""380 patent to Motamedi et al. discloses an integrated micro-electromechanical optical resonator that comprises a cantilever beam which is fixed to a substrate at one end and extends freely over the substrate at the other end. A bimorph actuator is also provided and is stacked on top of the beam at its fixed end. A reflective surface also partially covers the top of the beam at its free end. The bimorph actuator comprises material layers having different thermal expansion coefficients. A DC-biased AC voltage connected across the actuator causes it to heat and cool as the current passing through it increases and decreases. This creates a thermal bimorph effect which causes the cantilever beam and the reflective surface to oscillate in accordance with the varying current. Combining the resonator with a light source and actuator excitation circuitry creates an optical scanner engine which delivers a scan angle in excess of 20 degrees and a high scan rate. Unfortunately, the mirror surface provided on the cantilever beam of the ""380 patent may not have near diffraction-limited optical quality because the cantilever beam may become warped or otherwise distorted in response to the bimorph effect.
U.S. Pat. No. 5,867,302 to Fleming entitled xe2x80x9cBistable Micro-electromechanical Actuatorxe2x80x9d also discloses a MEM device having mirrors therein. In particular, the ""302 patent discloses a MEM actuator that is formed on a substrate and includes a stressed membrane of generally rectangular shape that upon release assumes a curvilinear cross-sectional shape due to attachment at a midpoint to a resilient member and at opposing edges to a pair of elongate supports. The stressed membrane can be electrostatically switched between a pair of mechanical states having mirror-image symmetry, with the MEM actuator remaining in a quiescent state after a programming voltage is removed. The bistable MEM actuator according to various embodiments of the present invention can be used to form a nonvolatile memory element, an optical modulator (with a pair of mirrors supported above the membrane and moving in synchronism as the membrane is switched), a switchable mirror (with a single mirror supported above the membrane at the midpoint thereof) and a latching relay (with a pair of contacts that open and close as the membrane is switched). Arrays of bistable MEM actuators can also be formed for applications including nonvolatile memories, optical displays and optical computing. FIGS. 7a-7b of the ""302 patent also disclose an embodiment of the MEM actuator that includes a rotatable mirror. Unfortunately, the process described in the ""302 patent for forming a membrane upon which the rotatable mirror is supported is relatively complicated and may not be suitable with conventional microelectronic processing techniques. Thus, the devices disclosed in the ""302 patent may not be readily integrated with electronic devices on conventional integrated circuit substrates.
Thus, notwithstanding the above-described MEM devices having mirrors therein, there continues to be a need for optoelectronic devices that can redirect optical beams and have near diffraction-limited optical quality and methods of forming and operating same that are compatible with conventional microelectronic device fabrication techniques.
It is therefore an object of the present invention to provide improved optoelectronic devices and methods of forming and operating same.
It is another object of the present invention to provide optoelectronic devices that can redirect optical beams and methods of forming and operating same.
It is still another object of the present invention to provide optoelectronic devices having movable reflective microstructures therein and methods of forming and operating same.
It is a further object of the present invention to provide optoelectronic devices having reflective microstructures therein with near diffraction-limited optical quality and methods of forming and operating same.
It is still a further object of the present invention to provide optoelectronic devices having optically reflective mirrors therein that can exhibit extreme flatness over apertures of up to several millimeters and methods of forming and operating same.
These and other objects, advantages and features of the present invention may be provided by optoelectronic devices that, according to one embodiment of the present invention, comprise a substrate having an opening therein that extends at least partially therethrough. A mirror having near diffraction-limited quality is also provided in the opening and is mechanically coupled to the substrate by a hinge so that the mirror can be rotated from a closed position to an open position. According to a preferred aspect of the invention, the mirror is formed from a monocrystalline silicon mirror backing layer having a thickness greater than about 10 xcexcm and an optically reflective mirror surface on the backing layer. The mirror surface may comprise gold or aluminum, for example, and may be applied to the backing layer using an evaporation or sputtering technique. This monocrystalline silicon mirror backing layer is highly resistant to warping or other distortions caused when stresses are applied to it.
According to another preferred aspect of the present invention, the hinge comprises a polycrystalline silicon hinge that provides a mechanical and an electrical connection to the substrate. A layer of metal such as nickel is also provided on a back surface of the mirror backing layer so that an application of a sufficiently strong magnetic field to the opening will induce a force on the layer of metal that operates to pull the mirror backing layer to an upright position. An electrostatic polysilicon clamp electrode is also provided on the monocrystalline silicon mirror backing layer. This clamp electrode can be used advantageously to clamp the mirror in a closed position even if a sufficiently strong magnetic field is applied to the opening. Thus, a substrate having a plurality of individually controllable reflective microstructures (e.g., xe2x80x9cpop-upxe2x80x9d mirrors) can be provided in accordance with the present invention.
According to another embodiment of the present invention, preferred methods of forming optoelectronic devices include the steps of forming an electrically conductive layer on a first surface of a substrate and then forming a mirror backing layer from the electrically conductive layer by forming an endless groove that extends through the electrically conductive layer. The endless groove is preferably formed using a deep reactive ion etching (DRIE) technique, however, an anisotropic etching step may also be performing using an etchant such as KOH. A portion of the substrate at a second surface thereof is then removed to expose a front surface of the mirror backing layer. An optically reflective mirror surface can then be formed on the front surface of the mirror backing layer. The substrate may also comprise a supporting layer of monocrystalline silicon (e.g., silicon wafer) having a thickness of greater than about 100 xcexcm and the step of removing a portion of the substrate may comprise the step of etching through the supporting layer of monocrystalline silicon using a deep reactive ion etching technique.
Methods according to the present invention also preferably include the step of forming a polysilicon hinge that is attached to a back surface of the mirror backing layer and is attached to the electrically conductive layer. The step of forming an optically reflective mirror surface also preferably comprises evaporating or sputtering a layer of gold or aluminum onto the front surface of the mirror backing layer. In order to enable magnetic actuation of the mirror backing layer, a layer of nickel is electroplated onto the back surface of the mirror backing layer. But, to inhibit magnetic actuation, the step of forming a polysilicon hinge also preferably comprises the step of forming a polysilicon electrostatic clamping electrode that is attached to the back surface of the mirror backing layer and overlaps the endless groove. The step of forming a polysilicon hinge and polysilicon clamping electrode is also preceded by the step of filling the endless groove with an electrically insulating layer by thermally oxidizing at least one sidewall of the mirror backing layer that is exposed by the endless groove. Any remaining opening in the groove may then be filled by depositing a phosphorus-doped silicate glass (PSG) layer into the groove and then planarizing the deposited layer to be flush with the back surface of the mirror backing layer.